1. Field of the Invention
The present invention is directed to an improved method of making rosin esters. Specifically, the invention provides an improved esterification process wherein rosin and a polyhydric alcohol are reacted in the presence of a phosphite ester and a phenol sulfide, as a combined catalyst. The phosphite ester/phenyl sulfide catalyst serves to accelerate and mediate the formation of rosin esters, resulting in esterified rosins which have numerous advantageous characteristics and which have particular utility as tackifiers in hot melt adhesive formulations.
2. Description of the Prior Art
It has long been known that gum, wood, tall oil, and other rosins can be chemically transformed to highly useful products by reaction with aliphatic alcohols in esterification processes. The various applications available for the products of such processes have led to the development of a number of procedures for carrying out these esterifications.
U.S. Pat. No. 3,780,013, for example, teaches an esterification reaction wherein a phenol sulfide compound is added in incremental amounts to tall oil rosin and penta-erythritol in the presence of a catalyst such as calcium hydroxide or zinc oxide.
U.S. Pat. No. 2,729,660 describes the use of esters of phosphorus acid (phosphite esters) as catalysts in the esterification of fatty acids or rosin acids with aliphatic alcohols. A disagreeable odor, however, has been observed in the resultant rosin ester.
U.S. Pat. No. 4,693,847 discloses the use of organic esters of hypophosphorus acid to catalyze the esterification of rosin with a polyhydric alcohol.
In U.S. Pat. No. 4,548,746, phosphinic acid is employed as a catalyst in an esterification of rosin with pentaerythritol. Base is then added to neutralize any residual phosphinic acid catalyst. The neutralized product, however, often promotes cloudiness when employed as an additive in hot melt adhesives, producing an undesirable hot melt result.
U.S. Pat. No. 4,650,607 teaches the use of phosphinic acid plus a phenyl sulfide in the esterification of rosin with a polyhydric alcohol. As with the '746 patent, the residual acid catalyst should be neutralized, and such neutralization causes a cloudy hot melt product.
U.S. Pat. No. 4,744,925 discloses the esterification of a rosin that has been modified by reaction with a phenyl, aldehyde and/or dicarboxylic acid prior to phosphinic acid/phenylsulfide catalyzed esterification with a polyhydric alcohol. Again, the reaction product must be neutralized, which, as discussed above, results in an undesirable cloudy hot melt product when used in such operations.
U.S. Pat. No. 4,725,384 teaches the use of a magnesium salt of a carboxylic acid, such as magnesium acetate, to neutralize the residual mineral acid catalyst in phosphinic acid catalyzed rosin and polyhydric alcohol esterification reactions. However, despite some improvements, the hot melt is still judged to be somewhat hazy.
The object of the present invention is to provide a novel method for the preparation of esters of rosin. It is a further object of the subject invention to employ a catalyst which accelerates the rosin esterification reaction rate and results in reduced reaction time. It is a still further object of the invention to provide a process which produces a rosin ester having particular utility as a tackifier in hot melt adhesive formulations. It is an even further object of this invention to provide a method of preparing esters of rosin exhibiting a color essentially equivalent to or lighter than the starting rosin.